Wireless communication systems are ubiquitous in many parts of the world. Several different Radio Access Technologies (RAT) have been deployed (e.g., WCDMA, GSM, LTE), and often different RATs cover the same geographic area(s). As users demand services and capabilities requiring higher bandwidth and interoperability, multi-standard systems, featuring simultaneous operation in two or more RATs, are being developed and deployed. FIG. 8 depicts a representative spectrum of a multi-standard radio system where the signals from different baseband sections are multiplexed together and sent through the same radio. In particular, consider a 10 MHZ LTE signal and a 5 MHZ WCDMA signal. The two signals can first be multiplexed and arranged next to each other in frequency, and the combined signal is then sent through the same radio. This concept can be extended to include additional baseband signals, and additional standards, such as GSM (as depicted in FIG. 8).
FIG. 9 depicts another approach to increased bandwidth—multi-carrier operation within a RAT. In this case, signals are modulated onto multiple carriers, using the same protocol. For example, FIG. 9 depicts the simultaneous transmission of 10 MHz and 5 MHz LTE signals. As used herein, a system with multiple carriers using the same standard (such as FIG. 9) is considered a special case of multi-standard system.
Amplifying circuits, including those in radio transceivers, are designed with a particular dynamic range. That is, the amplifier can faithfully reproduce signal variations within predetermined limits. Designing amplifiers with very large dynamic range is challenging, and adds costs and complexity to a system. However, the large dynamic range may be relatively rarely utilized, as signal peaks may be rare, compared to average signal values. A factor that reflects the prevalence of high signal peaks, as compared to average values, is the Peak to Average Power Ratio (PAPR), also known in the art as the crest factor. The PAPR may be formulated as
      C    =                                      x                          peak                    x                  r          ⁢                                          ⁢          m          ⁢                                          ⁢          s                      ,where|x|peak is the magnitude of a (positive or negative) peak value of a signal x, and xrms is the root mean square value of the signal. The PAPR is a dimensionless quantity, often expressed as dB. Each wireless communication protocol has characteristic PAPR limits required for reliable communication, which arise from the employed modulation scheme (e.g., QPSK=3.5-4 dB; 64QAM=7.7 dB; 128QAM=8.2 dB).
In multi-standard system, due to the aggregation of signals from different standards (and different modulation techniques), the problem of high PAPR is generally more pronounced than in the case of a single-standard system (with only one carrier). A high PAPR causes either non-efficient use of high power amplifiers in system transceivers, or high nonlinearity distortion in the transmitted signal. A common remedy is to limit the peak of the signal to achieve acceptable power efficiency. This causes distortion, both in-band and out-of-band. The in-band distortion is referred to as the error vector magnitude (EVM), which must be limited to a predefined value according to standards.
Among methods to reduce PAPR, the easiest ones in terms of implementation are methods that are transparent to the receiver. That is, the receiver does not need to know the method that is used in the transmitter, and no side channel is needed between the transmitter and the receiver. The simplest, and the most commonly used, transparent PAPR-reduction method is clipping the signal in the baseband section. Amplitude clipping limits the peak envelope of the signal to a predetermined value, but otherwise passes the signal through unperturbed. Clipping adds distortion, and introduces spurious signal harmonics. While the harmonics can be reduced by filtering the signal after clipping, the filtering may cause some peak re-growth, so that the signal again exceeds the clipping limit. Iteratively clipping and filtering the signal may minimize the induced noise, but at the cost of computational complexity and signal delay.
Tone injection is also commonly used to reduce PAPR. In tone injection, sine waves at specific frequencies (tones) are added as distortion on the top of the transmitted signal. Tone injection increases the modulation constellation size so that each of the points in the original basic constellation can be mapped into several equivalent points in the expanded constellation. Since each symbol in a data block can be mapped into one of several equivalent constellation points, these extra degrees of freedom can be exploited for PAPR reduction. This method is called tone injection because substituting a point in the basic constellation for a new point in the larger constellation is equivalent to injecting a tone of the appropriate frequency and phase in the multicarrier signal. See, for example, S. H. Han and J. H. Lee, “An overview of peak-to-average power ratio reduction techniques for multicarrier transmission,” IEEE Wireless Communication Magazine, vol. 12, pp. 56-65, April 2005, the disclosure of which is incorporated herein by reference in its entirety.
In a multi-standard system, different RATs have different requirements regarding the tolerated EVM and out-of-band radiation. Also in different power scenarios, constituent RATs may have different power spectral densities (PSD). When an aggregate, multi-standard signal is clipped for PAPR reduction, the resulting EVM and/or spectral mask of some of the RATs might be violated, while the others may not only tolerate the distortion, but could operate acceptably with even greater distortion. Also, in the case of OFDM-based RATs, within each carrier's bandwidth there might be uneven EVM or PSD requirements. This can be either due to different modulation schemes per subcarrier or boosting and deboosting of certain subcarriers. The EVM values for the constituent signals, resulting from conventional PAPR-reduction algorithms, are not adaptable and can not be weighted arbitrarily across different RATs.